1. Reflection and refraction
Last Time…
Total internal reflection
Reflection and refraction

2. Light rays and images Each point on object reflects light
Light propagates out, represented by rays perpendicular to wavefront.
Lens in our eye does some ‘imaging’ so that we identify origin of light rays.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

3. Question Does the fish appear Closer than actual Farther than actual
Same as actual
n=1.00
n=1.33
Virtual image
This is a kind of a lens. It creates an image by bending the rays. In this case the image is a virtual one. The rays diverge from a point, just like the original ones, but it is a different point.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

4. How do you ‘see’ this? Object Lens Image
Lens bends (refracts) light rays— forms image on retina
Sensors on retina report to brain.
Color information, intensity information
Here explain how eye lens makes a real image.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

5. Making a real image
Lens: Refracts light so that rays originating from a point are focused to a point on the other side.
Do demo of projecting light bulb filament on screen
This is a real image
Mon. Feb. 4, 2008
Physics 208, Lecture 4

6. How a lens works
Position surfaces to bend light rays in just the right way
Spherical surfaces are very close to the right ones. n1 n1
n2>n1
Optical Axis F
Do light board demo with lenses.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

7. Thin-lens approximation: Ray tracing
Optical Axis F
Image
Object F
1) Rays parallel to optical axis pass through focal point.
2) Rays through center of lens are not refracted.
3) Rays through F emerge parallel to optical axis.
Here image is real, inverted, enlarged
Mon. Feb. 4, 2008
Physics 208, Lecture 4

8. Different object positions
Image (real, inverted)
Object
Image (real, inverted)
Image (virtual, upright)
These rays seem to originate from tip of a ‘virtual’ arrow.
Here we use ray tracing to find where image location.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

9. Quick Quiz
I project a focused image onto a screen 2 meters away. I now want to make the image bigger without changing the lens. I should
Move screen farther away only
Move screen closer only
Move screen closer and object toward lens
Move screen farther and object toward lens
Move screen farther and object away from lens
Mon. Feb. 4, 2008
Physics 208, Lecture 4

10. Making an image s s’ Object Image How are all these related?
Image distance
Object distance f
focal length
How are all these related?
Use lens board to demonstrate how s’ changes when s changes.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

11. Your eye can change focal length
Lens
Object
Image
What is range of focal lengths if it can focus from near point (25 cm) to inf. onto retina 1.7 cm away?
Object at infinity:
Object at near point:
Very limited range
Mon. Feb. 4, 2008
Physics 208, Lecture 4

12. Image size vs object size: Magnification
Object height
Image distance
Object dist.
Image height
= M = Magnification
Mon. Feb. 4, 2008
Physics 208, Lecture 4

13. Question
At what object distance does image size equal object size (magnification=1)?
Object distance = f
Object distance = 2f
Object distance = f/2
Object distance = infinity
Object distance =0
Mon. Feb. 4, 2008
Physics 208, Lecture 4

14. Different object positions
Image (real, inverted)
Object
Image (real, inverted)
Image (virtual, upright)
These rays seem to originate from tip of a ‘virtual’ arrow.
Here we use ray tracing to find where image location.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

15. Virtual images Virtual image But rays can be focused by another lens
Image (virtual, upright)
These rays seem to originate from tip of a ‘virtual’ arrow.
objects closer to a converging lens than the focal length form a virtual image
Virtual image
can’t be recorded on film,
Can’t be seen on a screen.
But rays can be focused by another lens
e.g. lens in your eye (focus on retina)
e.g. lens in a camera (focus on film plane)
Mon. Feb. 4, 2008
Physics 208, Lecture 4

16. Do these rays come from real image, a virtual image, or an object?
Can’t tell. Rays are exactly equivalent, and can be imaged by a lens in exactly the same way.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

17. Magnifying glass o
Object at the near point - this is the biggest it will appear and be in focus
Object closer than focal point
Lens produces virtual image
Light rays appear to originate from virtual image
Virtual image is used as object for eye lens.
Have moved object ‘closer’, while permitting eye to focus
Image (virtual, upright) p q 
Can work this out. Suppose object is at near point = 25 cm.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

18. Image (virtual, upright)
Normal magnifier config, virtual image, upright
Image (real, inverted)
Image real, but eye cannot image it Blurry
3rd line: image is bigger than object, but not as big as just with simple magnifier. Ask how can we make it bigger? Answer: use a simple magnifier with the image as the object.
Image (real, inverted)
Eye can now form image of inverted lens image
Mon. Feb. 4, 2008
Physics 208, Lecture 4

19. Lens combinations Image of one lens acts as object for next.
Rays originate from image, whether real or virtual
Can then directly apply
Mon. Feb. 4, 2008
Physics 208, Lecture 4

20. Compound Microscope Real, inverted, image p q Virtual image ‘Object’
Eyepiece
Object
Objective
Object outside focal point Forms a real image
Real image used as object for eyepiece. Eyepiece forms virtual image for eye.
Mon. Feb. 4, 2008
Physics 208, Lecture 4

21. Far away objects
The moon is 3.8x108 m away, and 3.5x106 m diameter. I use a 1 m focal length lens to make an image of the moon. About what diameter is this image of the moon?
0.5 cm
1 cm
2 cm
10 cm
1 m
Not a very big image. How can
It be made ‘bigger’?
Mon. Feb. 4, 2008
Physics 208, Lecture 4

22. Telescope: object far away
Mon. Feb. 4, 2008
Physics 208, Lecture 4

23. Nearsightedness Object I can’t focus on this This, I can see
Mon. Feb. 4, 2008
Physics 208, Lecture 4

24. Fixing nearsightedness
Object
Mon. Feb. 4, 2008
Physics 208, Lecture 4